The R-T-B based permanent magnet (R represents a rare earth element, T represents Fe or Fe with part of it replaced with Co, and B represents boron) having the tetragonal compound R2T14B as the main phase is known to have excellent magnetic properties and has been a representative permanent magnet with high performance since it was invented in 1982 (Patent document 1: JP-A-59-46008).
The R-T-B based magnet in which the rare earth element R is formed of Nd, Pr, Dy, Ho and/or Tb is preferable as a permanent magnet material with a big magnetic anisotropy field Ha. Among them, the Nd—Fe—B based magnet having Nd as the rare earth element R is widely used because it has a good balance among saturation magnetization Is, curie temperature Tc and magnetic anisotropy field Ha, and is better in resources and corrosion resistance than R-T-B based magnets having other rare earth elements.
As a power unit used in people's livelihood, industries and conveyer equipments, the permanent magnet synchronous motor has always been utilized. However, the permanent magnetic synchronous motor in which the magnetic field generated by the permanent magnet is constant is hard to be driven as the induced voltage increases in proportion to the rotational speed. Thus, when the permanent magnetic synchronous motor is operated in an intermediate/high speed region or under a low load, a field weakening control via which the magnetic flux generated by the armature current is used to counteract the magnetic flux of the permanent magnet is needed to suppress the induced voltage below the supply voltage. As a result, a problem is there that the efficiency of the motor deteriorates.
In order to solve the technical problem mentioned above, a variable magnetic flux motor is developed by using a magnet in which the magnetic force changes reversibly via an external magnetic field (a variable magnetic force magnet). When the variable magnetic flux motor is operated in the intermediate/high speed region or under a low load, the decrease of efficiency in the motor due to the field weakening effect in the prior art can be inhibited by reducing the magnetic force from the variable magnetic force magnet.